AVAssetExportSession progress gets stuck on ipad but not on simulator

This piece of code works fine on the simulator. However, when I try to run the export on my iPad, it always hangs at progress value 0.14583-ish. Can somebody help me figure out why? been stuck on this for quite awhile.

Here is my code:

NSArray *compatibleP开发者_如何转开发resets = [AVAssetExportSession exportPresetsCompatibleWithAsset:composition];
if ([compatiblePresets containsObject:AVAssetExportPresetLowQuality]) {
    AVAssetExportSession *exportSession = [[AVAssetExportSession alloc]
                                           initWithAsset:composition presetName:AVAssetExportPresetLowQuality];


    exportSession.outputURL = [NSURL fileURLWithPath:[[ShowDAO getUserDocumentDir] stringByAppendingString:exportFilename]];
    exportSession.outputFileType = AVFileTypeQuickTimeMovie;

    CMTime start = CMTimeMakeWithSeconds(0, 1);
    CMTime duration = CMTimeMakeWithSeconds(1000, 1);
    CMTimeRange range = CMTimeRangeMake(start, duration);
    exportSession.timeRange = range;

    [exportSession exportAsynchronouslyWithCompletionHandler:^{
        switch ([exportSession status]) {
            case AVAssetExportSessionStatusCompleted:
                NSLog(@"Export Completed");
                break;
            case AVAssetExportSessionStatusFailed:
                NSLog(@"Export failed: %@", [[exportSession error] localizedDescription]);
                break;
            case AVAssetExportSessionStatusCancelled:
                NSLog(@"Export cancelled");
                break;
            default:
                break;
        }


    }];

    while(exportSession.progress != 1.0){
        NSLog(@"loading... : %f",exportSession.progress);
        sleep(1);
    }
    [exportSession release];

}

while(exportSession.progress != 1.0){
    NSLog(@"loading... : %f",exportSession.progress);
    sleep(1);
}

This while loop is blocking the main thread. The NSLog might not be able to fire properly. Try it without the while loop?